The Blues finished fifth last season, outside the Champions League places and 30 points adrift of Pep Guardiola's runaway champions.
However, Sane believes that the arrival of Sarri, whom Man City faced twice in the Champions League last season when he was in charge of Napoli, will rejuvenate the Stamford Bridge side.
"I think Chelsea [will be the biggest threat to Man City] because they have brought a really good coach," the Germany international told Sky Sports News.
"If you have seen how Napoli played last season, it was really, really attractive and really good. We really suffered against them.
"They will need some time to get really used to his tactics. They could be really tough. Also Liverpool too - they bought really good players. They could be really strong next season."
Sane was named PFA Young Player of the Year last season with 14 goals and 19 assists to his name across 49 Man City appearances in all competitions, but he was unexpectedly omitted from Germany's 23-man squad for the 2018 World Cup.